Currently available implantable cardiac rhythm management devices, including bradycardia and tachycardia pacemakers and cardiac defibrillators, have sense amplifier circuits for amplifying and filtering electrogram signals picked up by electrodes placed in or on the heart and which are coupled by suitable leads to the implantable cardiac rhythm management device. In most devices, the signals emanating from the sense amplifier are applied to one input of a comparator circuit whose other input is connected to a source of reference potential. Only when an electrogram signal from the sense amplifier exceeds the reference potential threshold will it be treated as a detected cardiac depolarization event such as an r-wave or a p-wave. The source reference potential may thus be referred to as a sensing threshold.
In the case of a programmable cardiac rhythm management device the prescribing physician can change the threshold potential of the comparator, but in spite of the flexibility which the programmable threshold offers, malsensing of cardiac depolarization will still occur frequently enough to result in patient discomfort and/or deleterious health effects. This may be due to the fact that cardiac depolarization events (intrinsic beats) can result in widely different peak amplitudes, depending on patient activity body position, drugs being used, etc. Lead movement and noise may further impede the detection of cardiac depolarization events. Noise sources may include environmental noise, such as 60 Hz power line noise, myopotentials from skeletal muscle, motion artifacts, baseline wander and T-waves. When the peak amplitudes associated with cardiac depolarization events become too small relative to a programmed threshold, or when noise levels in the electrocardiogram approach the sensing threshold, the likelihood of oversensing increases (i.e., false detection of depolarization events). If the sensing threshold is increased too high in an attempt to overcome the effects of noise, on the other hand, the likelihood of undersensing (i.e., failing to detect depolarization events) is increased. There is a need, therefore, for methods and apparatus that automatically adjust the sensing thresholds of cardiac rhythm management devices on a continuous beat-to-beat basis in a manner that better avoids both undersensing and oversensing.